A rotary hydraulic cylinder of the conventional nature has an inherent default that, when it is used at speeds of more than 5,000 r.p.m., the bearings of rotary shafts are usually overheated to cause the temperature of the whole device to rise and the capacity of such rotary cylinders to be impaired.
To prevent such a temperature rise in the prior art, it has been suggested: (1) the whole circulation volume of a working oil is increased to obtain natural cooling by increasing the capacity of an oil tank, (2) radiation cooling is obtained by promptly providing the heat generating section with fins and (3) a cooler is mounted on the hydraulic circuits of the working oil. Actually however, such equipment as mentioned above are large-sized or complicated in structure, thereby causing such equipment to be high-priced.